1. Field of the Invention
The present invention relates to implosion-resistant evacuated glass enclosures and more particularly relates to implosion-protective films for evacuated glass enclosures.
2. Brief Description of the Prior Art
Cathode ray tubes such as are used in television and other electronic display equipment contain a high vacuum. Because of this and because of their large size, breakage and subsequent implosion of television tubes causes the elements of the tube and fragments of glass to fly in all directions with great force.
Attempts have been made to alleviate this danger by providing an auxiliary safety shield consisting of a layer of tempered glass on the front of the tube. This has proven unsatisfactory because the safety shield does not protect the back of the tube which is susceptible to breakage when the tube is being installed and repaired. In addition, breakage from the front destroys the safety shield thereby removing any protection and even contributing to possible injury from flying glass. Also, dust and moisture can collect on the interior surface of the safety shield and on the face of the TV tube creating an optical fog which distorts the picture.
Another means to alleviate the problem of implosion is the use of so-called implosion shields positioned directly on the fact of the television tube. These implosion shields comprise a laminate structure of an outer layer of glass and an inner layer of cured unsaturated polyester resin laminated to the outer face of the television tubes. Unfortunately, television tubes with implosion shields are very costly to make and, in addition, the implosion shield only offers protection to the face of the tube and does not protect the back areas of the tube which are also susceptible to breakage during installing and repair services. An example of the laminate implosion shields is disclosed in U.S. Pat. No. 3,265,234 to McGary, Jr. et al.
Electric lamps are also manufactured under vacuum. The vacuum is of varying degrees and even when not as high, implosion and flying glass is still a danger when the lamps are broken. It is known in the art that electric lamps can be coated with elastomeric silicone resins to make them shatter resistant. Unfortunately, these elastomeric silicone resin coatings are tacky and attract dust which adheres firmly and is not easily removed. This darkens the lamp and reduces its light output. Glossy harder silicone coatings are not effective in reducing shattering.
U.S. Pat. No. 3,715,232 discloses a composite coating for electric lamps comprising a shatter-resistant silicone rubber coating and a hard silicone coating over that to resist the accumulation of dust. Although this multiple coating is reported as providing both shatter resistance and a very glossy surface which will not attract dust, the cost of manufacture is somewhat expensive because the silicones are expensive and two coatings have to be applied.
The present invention overcomes many of the shortcomings of the prior art. The present invention relates to an implosion-resistant polyurethane film which is applied to evacuated glass enclosures such as television tubes and electric lamps. The implosion-resistant film overcomes the problems associated with implosion shields and implosion laminates for television tubes. The film can be easily applied to all areas thereby offering protection not only to the viewing area but also to the area behind the tube. In addition, the film can be applied by conventional techniques such as brushing and spraying with a polyurethane coating composition or by lamination or heat shrinking with a preformed film, thereby providing an economical way to provide implosion protection. The film is non-tacky and can be applied in one layer with good implosion protection thus providing economic incentives over the multiple silicone coatings described in U.S. Pat. No. 3,715,232. Polyurethane coatings also have good abrasion resistance, i.e., resistance to scuffing and scratching that cause distortion in images.